Time of flight ranging systems, are commonly used in level measurement applications, and referred to as level measurement systems. Level measurement systems determine the distance to a reflector (i.e. reflective surface) by measuring how long after transmission of a burst of energy pulses, an echo is received. Such systems typically utilize ultrasonic pulses, pulse radar signals, or microwave energy signals.
Time of flight ranging systems are commonly utilized in remote locations where process variable data is transmitted to a central location for further processing or collection. A common means for transmitting such data is by a current loop. The value of the process variable is represented by the magnitude of a current passing through the loop, with the magnitude lying between predetermined minimum and maximum values, typically 4 mA and 20 mA. Such a current loop has a high degree of noise immunity and has gained widespread industrial acceptance.
In time of flight systems, the transmitter usually has electrical power requirements of its own, and it is often convenient to meet these power requirements from the current passing in the loop. A limitation of such loop powered transmitters has been that they must be able to operate at the minimum level of loop current, typically 4 mA. In recent years, a number of “smart” or “intelligent” transmitters have been developed, which utilize microprocessors or microcontrollers to control sensing or measurement of the process variable, and conversions of the data generated into an appropriate current level in the loop.
U.S. Pat. No. 5,416,723 which is issued on May 16, 1995 to the common assignee of the subject application disclosed such a “loop powered two wire intelligent process variable transmitter”. The loop powered intelligent transmitter includes a microprocessor, a memory for storing a program for execution by the microprocessor, circuit elements for measuring a process variable under control of the microprocessor in accordance with the stored program, a current control circuit controlled by the microprocessor and for determining amplitude of a current passing in a current loop between maximum and minimum finite values in a predetermined relationship to a measured value of the process variable, and a power regulating circuit providing power at a controlled potential required by the microprocessor and the measuring circuit elements. According to the invention, the regulating circuit is associated with a circuit configured to sense a deficit in its capability to supply the integrated power requirements of the microprocessor and measuring circuit elements, and to delay the execution of the stored program sufficiently in response to the sensing of such a deficit to reduce the integrated power requirements to overcome the deficit. According to one arrangement, a microprocessor is utilized which has, in addition to its normal operating mode, a low power consumption “sleep” mode in which program execution is halted. A power deficit results in halting the program execution, and hence of measurement processes controlled by the microprocessor, until the deficit is made up, such that the program executes intermittently, the extent of the intermittence depending on the extent to which the normal operating power requirement exceeds the available power. In an alternative arrangement, a microprocessor is utilized of a type whose power consumption is proportional to its clock rate, e.g. most CMOS microprocessors, and which can operate satisfactorily over a wide range of clock rates, and the clock rate is reduced from a normal maximum value in response to a power deficit condition. The first arrangement is preferable where certain operation controlled by the microprocessor must be carried out in real time.
The arrangement disclosed in U.S. Pat. No. 5,416,723 is effective in storing energy for future use by the microprocessor and circuitry, for example, when there is less power available from the current loop then required for the circuitry, and allows the power from the current loop to exploited more effectively. However, in situations where the current loop provides more power then is required by the circuitry, the excess power is dissipated as heat. It will be appreciated that it is advantageous to utilize this extra capacity in a form other than just dissipation as heat. Accordingly, the present invention provides a circuit arrangement for more efficiently utilizing the excess power which may be available from the current loop.